1. Field of the Invention
This invention relates to an exposure control system and a motion picture camera using the same.
2. Description of the Prior Art
The prior art has proposed two systems for controlling the exposure of a motion picture camera adapted to make exposures not only at normal light levels but also at light levels outside the normal light range, and particularly of a motion picture camera adapted to make exposures even at low light levels. According to one system, the exposure control is made by manually adjusting the angular positions of one of the paired shutter blades relative to the other and the diaphragm means defining the size of the aperture opening through which an image forming light beam passes. According to another system, after the camera diaphragm is adjusted to a predetermined setting, the exposure time interval is controlled by varying the speed of rotation of the shutter in conformance with the preselected aperture value. These prior art systems, however, have various problems. In the case of the former system, because of the incapability of automatic exposure control, the photographer has to manually control the size or angle of an opening defined by the shutter blades in accordance with the object brightness level. When fast changes in light conditions occur in the scene being photographed, or when the camera is rapidly aligned from one object to another with a change in brightness level, the necessary manipulation of the camera becomes very troublesome or almost impossible. Another problem is that the quality of reproduced motion pictures or photographs is often dependent upon the ability of the photographer to evaluate the light conditions or object brightness levels. Still another problem arising from the particular shutter structure is that the range of available exposure intervals is relatively limited, depending upon the range of adjustment of the angular positions of the shutter blades because of an assurance that the shutter serves to completely block the path of an image forming light passing through the maximum size of the diaphragm aperture. A furthermore problem is that the shutter must be provided with mechanical adjusting means for adjusting the angular positions of the shutter blades for the purpose of utilizing the shutter as the exposure time control means, the provision of such adjusting means making the camera structures complicated. This complexity will in turn cause an increase in dimensions of the camera body and the probability of occurrence of mechanical damages. As is known in the art, recently the requirements for minimization of the weight and bulk of the camera and for the reduction of the production cost become of more and more importance from the multi-purpose camera systematization view point. Under such a situation, it is of ever-increasing necessity to construct the camera body with an automatic exposure control apparatus in mechanically simple form by use of a reasonable electrical circuitry.
In the case of the latter exposure control system, the speed of rotation of the shutter is varied with variation of light condition in the scene being photographed, or with variation of object brightness level as the camera is aligned from one to another object of different brightness whereby the time-spaced relation between the successive exposures is made quite uncertain. This is not suited for the availability of accurate recording data. Further, when the camera is switched to the normal operation position, the shutter is rendered operative only at a preselected rotation speed. With the latter system, therefore, it is often possible to overlook a necessary manupulation when switching the camera between the normal and low light level exposure control ranges.
It is also known in the art to provide motion picture cameras of the type in which the shutter and film transport mechanisms are driven by a single small electric motor and are braked in such a manner that, upon release of the shutter actuation member, the power supply circuit for the motor is opened and then the shutter stopping member is brought into engagement with the control shaft being driven by the motor, whereby the shutter is held from rotation in an angular position. With such a driving and braking mechanism, however, it is often possible to stop the shutter blade in an aperture-unblocking position because the shutter driving motor continues to rotate under the action of only its inertia after the supply circuit is opened. In such a case, a so-called "blank frame" is produced.